Find string which is lexically sorted mid point between two other strings

findLexicalMiddleString.ts

// Algorithm adapted from an amazing StackOverflow answer:
// https://stackoverflow.com/questions/38923376/return-a-new-string-that-sorts-between-two-given-strings/38927158#38927158

const A = 97;
const B = 98;
const Z = 122;
const START = A - 1;
const END = Z + 1;

function findLexicalMiddleString(prev: string, next: string) {
  let p = START;
  let n = END;
  let pos = 0;

  // find leftmost non-matching character
  do {
    p = pos < prev.length ? prev.charCodeAt(pos) : START;
    n = pos < next.length ? next.charCodeAt(pos) : END;
    pos += 1;
  } while (p == n);

  // copy identical part of string
  let str = prev.slice(0, pos - 1);

  if (p == START) {
    // prev string equals beginning of next

    // next character is 'a'
    while (n == A) {
      // get char from next
      n = pos < next.length ? next.charCodeAt(pos++) : END;
      // insert an 'a' to match the 'a'
      str += "a";
    }

    // next character is 'b'
    if (n == B) {
      // insert an 'a' to match the 'b'
      str += "a";
      // set to end of alphabet
      n = END;
    }
  } else if (p + 1 == n) {
    // found consecutive characters

    // insert character from prev
    str += String.fromCharCode(p);
    // set to end of alphabet
    n = END;

    // p='z'
    while ((p = pos < prev.length ? prev.charCodeAt(pos++) : START) == Z) {
      // insert 'z' to match 'z'
      str += "z";
    }
  }

  // append middle character
  return str + String.fromCharCode(Math.ceil((p + n) / 2));
}